Minimum Alveolar Concentration (MAC) was introduced in 1964 as a standard of assessing the depth of anesthesia. It is defined as an anesthetic concentration in the alveolar that prevents a response to a painful stimulus in 50% of subjects. Such a definition does not incorporate the time required for an anesthetic concentration in the brain to reach equilibrium with the anesthetic concentration in the alveolar. See Lin, C. Y. (1994) Anesth Intens Care 22: 362–373. Thus, the MAC standard cannot be used to accurately assess the depth of anesthesia.
Practically, most anesthesiologists determine the depth of anesthesia based on exterior symptoms in the patient, such as ocular myosis or abnormal blood pressure. However, these symptoms vary from patient to patient, and reliance on them inevitably involves subjective judgment of the anesthesiologists. Thus, other methods have been developed. For example, one method includes repeatedly administering stimuli to a patient, recording the patient's electrical brain activity after each stimulus, and transforming the record as an indication of the depth of anesthesia. See International Patent Publication No. WO 91/19453.
There remains a need for an objective method for assessing the depth of anesthesia.